1. Field of the Invention
The present invention relates to a metal oxide for use as a catalyst support of a catalyst for purifying exhaust gases discharged from motor vehicles, and a catalyst using such metal oxide. The metal oxide in accordance with the present invention may be used as a catalyst support of a catalyst for oxidizing diesel particulates, a solid electrolyte, an electrode material, ceramics-dispersion reinforced particles, an ultraviolet ray-shielding material or the like.
2. Description of Related Art
The catalyst for purifying exhaust gases (three-way catalyst) is composed of a substrate which is made of a heat-resistant ceramics such as cordierite, a catalyst-support layer which is formed on the substrate, and is made of an active alumina or the like, and a catalyst metal such as Pt, which is supported by the catalyst-supporting layer. This three-way catalyst oxidizes hydrocarbon (HC) and carbon monoxide (CO) which are contained in exhaust gases from internal combustion engines, and reduces nitrogen oxides (NOx) which are contained in the exhaust gases, thereby purifying exhaust gases.
The oxygen concentration in exhaust gases, however, greatly varies due to the operating condition of the internal combustion engines. Consequently, the purifying activity, that is oxidizing and reducing activity, of the three-way catalyst may become instable. To prevent this phenomenon, ceria (hereinafter referred to as CeO2) has been added to the catalyst-supporting layer as a promoter. CeO2 has an oxygen storage capacity (hereinafter referred to as OSC) of storing oxygen in an oxidization atmosphere, and releasing oxygen in a reduction atmosphere. By virtue of the OSC of CeO2, a stable purifying activity which does not vary due to the variation of the oxygen concentration in exhaust gases can be obtained. In order to enhance the OSC, it is desired to enlarge the specific surface area of CeO2. Accordingly, CeO2 in a powder state has been used.
It has been reported that where the three-way catalyst which contains a catalyst metal and CeO2, is used at elevated temperatures of 800° C. or more, the OSC thereof may decrease due to the crystal growth of CeO2. To restrain the crystal growth of CeO2 and maintain a high OSC, means of adding zirconia (hereinafter referred to as ZrO2) or oxides of rare earth elements except for cerium has been developed. By adding ZrO2 or the like to CeO2, the heat resistance of CeO2 is improved, thereby improving the OSC thereof after endurance tests at elevated temperatures.
Japanese Unexamined Patent Publication (KOKAI) No. 04-055315, for example, discloses the method for producing fine powders of cerium oxide, which includes the steps of co-precipitating a CeO2 precursor and a ZrO2 precursor from a mixture aqueous solution of a water-soluble salt of cerium (Ce) and a water-soluble salt of zirconium (Zr), and subjecting the obtained co-precipitate to a heat treatment. With this method, CeO2 and ZrO2 become a composite oxide by the heat treatment of the obtained co-precipitate, thereby forming an oxide solid solution in which CeO2 and ZrO2 are dissolved in a solid phase.
Japanese Unexamined Patent Publication (KOKAI) No. 08-215569 discloses the technique of using a CeO2—ZrO2 composite oxide which is prepared from metal alkoxides. In the CeO2—ZrO2 composite oxide prepared from the metal alkoxides by the sol-gel method, Ce and Zr are made composite on an atomic level or molecular level, and become a solid-solution. Accordingly, the heat resistance is improved, and a high OSC is ensured from a beginning of use of CeO2—ZrO2 composite oxide to the end of an endurance test.
Published Japanese Translation Publication (KOHYO) No. 10-512191 discloses a CeO2—ZrO2 solid solution which is prepared by making the pH of a mixture aqueous solution of a water-soluble inorganic cerium salt and a water-soluble inorganic zirconium salt alkaline, thereby co-precipitating precursors thereof, and calcining the co-precipitated precursors. The CeO2—ZrO2 solid solution produced by this co-precipitation method exhibits a high specific surface area even after calcining at temperatures as high as 1000° C., and accordingly, is most suited to the catalyst support.
However, recently, the temperature of the exhaust gases from motor vehicles tends to increase, and accordingly, even the catalyst using the above-described conventional CeO2—ZrO2 solid solution has the problem that the heat resistance is insufficient, though OSC is high. It becomes clear that this problem is caused by the decrease of the specific surface area due to the occurrence of sintering in the CeO2—ZrO2 solid solution.
In addition, in the case of the conventional CeO2—ZrO2 solid solution, ZrO2 is not highly dissolved in a solid phase, OSC thereof is merely about 150 μmol-O2/g. The performance is inferior. Even where ZrO2 is highly dissolved in a solid phase, a high OSC is not always achieved thereby. Japanese Unexamined Patent Publication (KOKAI) No. 09-221304, for example, discloses a CeO2—ZrO2 solid solution with a high solid-solubility. In the CeO2—ZrO2 solid solution of this publication, however, the ratio of cerium ions which contribute to the achievement of the OSC is 53% where the atomic ratio of Ce/Zr is 5/5, and 69.5% where the atomic ratio of Ce/Zr is 3/7. The using efficiency of cerium ions is not sufficient. Accordingly, it is desired to further improve the OSC by further increasing the using efficiency of cerium ions.
On the other hand, Japanese Unexamined Patent Publication (KOKAI) No. 08-109020 discloses a composite oxide containing cerium oxide, zirconium oxide and hafnium oxide, and having a φ′ phase. The φ′ phase indicates a CeO2—ZrO2 solid solution in which cerium ions and zirconium ions are regularly arranged.
It is known that the CeO2—ZrO2 solid solution which has a regular arrangement of cerium ions and zirconium ions, that is correctly a CeO2—ZrO2 composite oxide, can exhibit OSC which is as high as about 800 μmol-O2/g even when the ambient temperature increases up to 1000° C. However, in order to form a regular arrangement in the composite oxide disclosed in the above-described publication, a reduction treatment at elevated temperatures is needed, and accordingly, the composite oxide disclosed in the above-described publication exhibits the problem that the specific surface area decreases to about 5 m2/g due to sintering.
Japanese Unexamined Patent Publication (KOKAI) No. 09-221304 also discloses a method for producing a CeO2—ZrO2 solid solution, wherein a surfactant is added upon precipitating oxide precursors from an aqueous solution in which metal salts of Ce and Zr are dissolved, to decrease the diameter of crystallites, thereby increasing the solid-solubility and enhancing the exhibited OSC. And, Published Japanese Translation Publication (KOHYO) No. 2001-524918 also discloses a method for producing ZrO2 or a CeO2—ZrO2 solid solution, wherein a surfactant is added upon precipitating oxide precursors from the aqueous solution in which metal salts are dissolved, to suitably adjust the particle size distribution, the pore volume, or the like.
With the addition of the surfactant upon precipitating the oxide precursors, as disclosed above, plural kinds of precipitated particles are captured in micelles of the surfactant homogeneously. And neutralization, coagulation and maturation proceed in the micelles, and consequently, the formation of particles of the solid solution proceeds in narrow spaces in which plural components are included homogeneously and concentrated. In addition, with the dispersion effect of the surfactant, the dispersion properties of the precipitated particulates are improved, and the segregation thereof is reduced to enhance the contacting degree thereof. Consequently, the precipitated particulates become highly dissolved, and the mean diameter of the crystallites can be decreased.
The method by which a surfactant is added upon precipitating the oxide precursors from an aqueous solution of metal salts, however, has the problem that as the diameter of secondary particles of the obtained oxide powder decreases, the pore volume may be also decreased, and consequently, where used as a catalyst support, the properties of the obtained oxide powder may not be achieved sufficiently.
For example, the catalyst for purifying exhaust gases, which has a honeycomb substrate and a coat layer composed of a catalyst support powder, and supports catalyst components such as noble metals in the coat layer, has the problem that the catalyst components in a lower part of the coat layer cannot be used effectively. This is considered to be caused by the pore volume being small.